ui-dropdown/vendors/popper/docs/_includes/example50-code.html

A custom dropdown element

<h2 id="example5" tabindex="0">Viewport boundaries</h2>
<p>
    By default, poppers use as boundaries the page viewport.<br>
    Scroll the page to see the popper flip when hits the page viewport margins.
</p>

{% highlight javascript %}
var popper = new Popper(referenceElement, onBottomPopper, {
    placement: 'bottom'
});
{% endhighlight %}